The overall goal of the proposed research is to test the internal and external validity of subtypes of ADHD using converging methods. To accomplish these objectives as part of our ongoing study, we will administer a batter of neuropsychological and psychiatric measures to samples of RD, ADHD, and control twin pairs (50 MZ and 50 DZ pairs in each group) and their siblings. The reliability and internal validity of the ADHD subtypes will be tested by examining inter-rater agreement and test-retest reliability and conducting factor and cluster analyses of individual symptoms. Diagnostic and discriminant validity of the subtypes will be tested by comparing subtypes on measures of functional impairment and other clinical correlates. Etiological relations among the subtypes will be tested using multiple regression analyses of twin data and molecular genetic linkage and association methods, and the cognitive relations among the subtypes will be tested with neuropsychological measures. The neuropsychological battery is designed to test further the executive deficit hypotheses of ADHD by pitting it against competing hypotheses that suggest ADHD is attributable to a deficit in motivation, arousal regulation, or response modulation.